Just to follow up, I'm also using a T-Mobile V10 on AT&T, and I get full LTE, best signal of any phone I've had actually. These are my APN settings:
NAME: AT&T
APN: pta
Proxy: (not set)
Port: (not set)
Username: (not set)
Password: (not set)
MMSC:
http://mmsc.mobile.att.net
MMS Proxy: proxy.mobile.att.net
MMS port: 80
MCC: 310
MNC: 410
Authentication Type: Not set
APN type: default,mms,supl,hipri,fota
APN protocol: IPv4/IPv6
Keep in mind that with the V10, you first have to unlock the APN settings by accessing a hidden menu through your dialer. You've probably already done this to set up your Cricket SIM, but just in case anyone else needs the steps, you open the dialer, dial 277634#*#, find "APNUnlock", click that, then click "Unlock APN", restart your phone, then you can set a new APN in the regular mobile network settings.